A super-heterodyne is a widely used and well-proven radio receiver architecture. It can sufficiently eliminate interfering signals while selecting the desired signal. Two or more local oscillators (LO) convert an RF input signal to two or more Intermediate Frequencies (IFs), where IF=LO-RF, before it reaches the baseband frequency. The first IF is much higher in frequency (usually at least twice) than the bandwidth of the RF hyperband (The hyperband consists of many channels. The receiver should be capable of selecting any one channel in the hyperband on command). For instance, a typical receiver for North American cellular band 869 to 894 MHz, i.e., 25 MHz wide, and has a first IF at higher than 50 MHz. A receiver for Personal Communication Services (PCS) 1930 to 1990 MHz band, i.e., 60 MHz wide, has a first IF at higher than 120 MHz. Such IFs ensure a sufficient removal of the spurious signals at LO+IF and at LO-1/2IF, the so called "half IF spurious response". After bandpass filtering, the signal at first IF is then down-converted to a lower second IF, typically around 450 KHz or 10.7 MHz for further filtering (channel selection). Then, the IF signal is demodulated to baseband, i.e., a center frequency of 0 Hz.
It is desirable to simplify the radio receiver design by eliminating the second IF and down-converting directly to baseband from first IF. However, this is not possible when the channels are narrow (say 30 kHz) and the IF is high enough to avoid the 1/2 IF spurious response and image frequency. The reason is that the ratio of 120 MHz to 30 kHz is 4000, and so a filter with a Q (quality factor) of 4000 and a center frequency of 120 MHz would be required for channel selection. Such a filter is expensive and extremely difficult to realize.